Long-period gratings typically comprise a length of fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA.. In contrast with conventional Bragg gratings, long-period gratings utilize a periodic spacing .LAMBDA. which is typically at least ten times greater than the transmitted wavelength .lambda., i.e. .LAMBDA.&gt;10 .lambda.. The key distinguishing property of a long-period grating is its ability to couple light from one optical mode of a fiber to another mode propagating in the same direction, with very low back-reflection. (This is in contrast with short-period Bragg gratings that couple to back-propagating modes and hence, reflect light.) Specific examples of these modes include coupling from a guided mode to a non-guided mode, from one guided mode to another guided mode, or from one polarization mode to another polarization mode. A non-guided mode is a spatial mode that is not defined by the core alone but rather by the entire fiber structure. Usually the non-guided mode is a cladding mode, a leaky mode or, in the case of a multiple-layered cladding, a ring mode.
Long-period gratings that couple light from the guided mode to a non-guided (cladding) mode are useful as wavelength-dependent loss elements. For example, these gratings are used for broadband high power sources (C. W. Hodgson, et al., 9 Optical Society of America Technical Digest Series, Paper TuG3 (1996)), as gain-equalizers in optical amplifiers, (Vengsarkar et al., 21 Optics Letters, 336 (1996)), as noise-rejection filters in erbium doped amplifiers (Vengsarkar et al. 14, J. Lightwave Technology, 58 (1996)) and in cascaded high-power Raman lasers (S. G. Grubb and A. J. Stentz, Laser Focus World, p. 127 (February 1996)).
In each instance, the wavelength range over which efficient transfer of optical power takes place defines the utility of the device. While many such devices based on long-period gratings have been designed and demonstrated, the bandwidth of operation has usually been restricted to 5-10 nm. This range is severely restricting, especially since the useful erbium amplifier bandwidth is about 35 nm. One method of increasing the fiber grating bandwidth is to chirp the grating so that the period .LAMBDA. varies along the length. While chirping allows some increased bandwidth the increase is usually at the expense of a reduction in the extent of mode-conversion. Hence, there is a need for long-period gratings having wider bandwidths.